In recent years, in mobile communication devices represented by cellular phone or in net-work related electronic devices such as server, router, etc., it is required to transmit and treat huge amount of information with low loss and high speed, so that electric signals treated on the printed wiring board also become high frequencies. However, when a frequency is higher, electric signals likely decreased, so that it is required to use a material having low transmission loss for a printed wiring board to be used in these fields of art. That is, in a high frequency band region of 1 GHz or higher, a material having excellent dielectric characteristics represented by low values in dielectric constant and dielectric dissipation factor is required to be used.
In these backgrounds, it has been attracted attention to use a cyanate ester resin having excellent dielectric characteristics of a cured product in a resin composition for printed wiring board. Heretofore, it has been proposed a resin composition in which a generally used epoxy resin such as a bisphenol A type epoxy resin, a brominated bisphenol A type epoxy resin, a phenol novolac type epoxy resin and a cresol novolac type epoxy resin is formulated with a cyanate ester resin, and it has been known that humidity resistance or heat resistance under humidity are improved than the case of a cyanate ester resin alone (for example, Japanese Patent Publication No. Sho. 46-41112, Japanese Unexamined Patent Publications No. Sho. 50-132099 and No. Sho. 57-143320). However, these resin compositions are inferior in dielectric characteristics than those in which no epoxy resin is formulated due to the effect of an epoxy resin.
Moreover, according to the present inventors, a resin composition in which dielectric characteristics at high frequency band region is improved has been proposed as compared to the case where the above-mentioned generally used epoxy resin is utilized by selecting, as an epoxy resin, a specific epoxy resin such as a naphthalene skeleton-containing epoxy resin, an aralkylene skeleton-containing epoxy resin, a lower alkyl group substituted phenol salicylic aldehyde novolac type epoxy resin, and a dicyclopentadiene skeleton-containing epoxy resin, and formulating them with a cyanate ester resin (for example, Japanese Unexamined Patent Publications No. Hei. 8-176273, No. Hei. 8-176274 and No. Hei. 11-60692).
Also, a resin composition having sufficient dielectric characteristics even when an epoxy resin is formulated has been proposed by the present inventors by modifying a cyanate ester resin with a specific monovalent phenol compound to prepare a phenol-modified cyanate ester resin composition, whereby dielectric characteristics possessed by a cyanate ester resin have been more improved (for example, Japanese Unexamined Patent Publication No. 2001-240723).
However, in either of the above-mentioned resin compositions, humidity resistance or heat resistance of a cured product is improved than the case where a cyanate ester resin or a modified cyanate ester resin is used alone, but due to the effect of an epoxy resin, a dielectric constant and a dielectric dissipation factor at high frequency band region are increased, or stability of dielectric characteristics relative to temperature is observed to be lowered (for example, increase in drift accompanied by temperature change of dielectric characteristics), so that the problems remain in the point of improving dielectric characteristics.
In particular, in a multi-layered printed wiring board to be used for wireless base station equipment use or high speed server, router, etc., temperature of the printed wiring board during operation of the device sometimes becomes a high temperature of 85 to 90° C., whereby there are possibility of causing significant problems that a dielectric constant is fluctuated due to change in temperature, or mismatching in impedance or transmission error accompanied by increase in transmission loss occurs when a dielectric dissipation factor becomes high. Accordingly, necessity for a resin composition for printed wiring board excellent in dielectric characteristics including temperature-dependency has been increased.
Also, in such background, as a resin composition for printed wiring board, a resin composition in which a cyanate ester excellent in dielectric characteristics and a polyphenylene ether are mixed and kneaded has been proposed (for example, Japanese Patent Publication Sho. No. 61-18937). However, in these resin compositions, when a formulation amount of the cyanate ester is larger, a dielectric dissipation factor tends to be high relative to the value of the dielectric constant. On the other hand, when a formulation amount of the polyphenylene ether is increased to lower the dielectric dissipation factor, a melting viscosity of the resin composition becomes high whereby flowability thereof becomes insufficient, so that there are problems that molding property is worsened.
Also, a resin composition in which an epoxy resin such as a bisphenol A type epoxy resin, a brominated bisphenol A type epoxy resin, a phenol novolac type epoxy resin and a cresol novolac type epoxy resin is formulated to a cyanate ester resin and a polyphenylene ether has been proposed (for example, see Japanese Patent Publication No. Hei. 4-57696). However, in these resin compositions, there are problems that dielectric characteristics at high frequency band region do not yet satisfy with a required level due to the effects of components other than the cyanate ester resin and the polyphenylene ether. In particular, in a multi-layered printed wiring board to be used for wireless base station equipment use or high speed server, router, etc., temperature of the printed wiring board during operation of the device sometimes becomes a high temperature of 85 to 90° C., whereby there are possibility of causing significant problems that a dielectric constant is fluctuated due to change in temperature, or mismatching in impedance or transmission error accompanied by increase in transmission loss occurs when a dielectric dissipation factor becomes high. Accordingly, necessity for a resin composition for printed wiring board excellent in dielectric characteristics including temperature-dependency has been increased.
Also, a resin composition excellent in heat resistance, molding property and workability as well as having good dielectric characteristics at high frequency band region has been proposed by the present inventors by modifying a cyanate ester resin with a specific monovalent phenol compound to prepare a phenol-modified cyanate ester resin composition, whereby dielectric characteristics possessed by a cyanate ester resin have been more improved, and further a polyphenylene ether resin is formulated (for example, Japanese Unexamined Patent Publications No. Hei. 11-21452 and No. Hei. 11-21453). However, in these resin compositions, it has been required to further improve humidity resistance under sever conditions such as a pressure cooker test for a long period of time, etc. while maintaining good dielectric characteristics including temperature-dependency.
The present invention has been accomplished in view of the above circumstances, and an object thereof is to provide a resin composition for printed wiring board which has the same molding property and workability as those of a thermosetting resin material such as an epoxy resin, etc., and having excellent humidity resistance and heat resistance, and also shows particularly excellent dielectric characteristics at high frequency band and excellent stability of dielectric characteristics against change in temperature, and a varnish, a prepreg and a metal clad laminated board using the same.
In the first invention of the present invention, the present inventors have earnestly studied and as a result, they have found that a resin composition for printed wiring board which is improved in humidity resistance and has excellent dielectric characteristics at high frequency band, is a little in drift property against temperature change in dielectric characteristics, and shows an excellent stability, by using an epoxy resin having a biphenyl structure (hereinafter referred to as an epoxy resin containing a biphenyl structure (Component (B))) in the molecule as at least one kind of an epoxy resin when an epoxy resin is formulated to a cyanate ester resin (Component (A)), to accomplish the present invention.
In the first invention of the present invention, in a cyanate ester resin composition into which the conventionally employed epoxy resin is formulated, an isocyanuric ring, an oxazolidinone ring, etc. which have higher porality than a triazine ring are formed other than the triazine ring, so that it can be considered that dielectric characteristics (in particular, dielectric dissipation factor) are worsened. On the other hand, in the resin composition containing an epoxy resin containing a biphenyl structure of the present invention, a biphenyl group show hydrophobic property and low polarity, so that it can be considered that bad effects caused by using an epoxy resin in combination on dielectric characteristics are reduced than the conventional one. Also, a biphenyl skeleton with a brittle structure is introduced into the cured product, so that it can be considered that a temperature-dependency of dielectric characteristics is a little due to low mobility of a molecular chain particularly at a high temperature region.
Moreover, the resin composition for printed wiring board of the first invention of the present invention is excellent in heat resistance at the time of absorbing moisture and higher strength and elongation at a glass state region or elongation at a high temperature region as compared with the case where a cyanate ester resin, etc. is used alone or the case where the conventional epoxy resin is used in combination with a cyanate ester resin, so that it is preferred for the uses such as a laminated board to be used for a multilayered printed wiring board of ten-layers or more to which crack resistance at the time of drilling processing or reflow, and severe heat resistance and for a prepreg.
In the second invention of the present invention, the present inventors have carried our earnest studies, and as a result, they have found that a resin composition for printed wiring board which can ensure excellent humidity resistance, has excellent dielectric characteristics at high frequency band, is a little in drift property against temperature change in dielectric characteristics, and shows an excellent stability, by using an epoxy resin having a biphenyl structure (hereinafter referred to as an epoxy resin containing a biphenyl structure (Component (d))) in the molecule as at least one kind of an epoxy resin when an epoxy resin is formulated to a resin composition containing a cyanate ester resin (Component (A)), a monovalent phenol compound (Component (b)) and a polyphenylene ether resin (Component (c)), to accomplish the present invention.
According to the second invention of the present invention, in a cyanate ester resin composition into which the conventionally employed epoxy resin is formulated, an isocyanuric ring, an oxazolidinone ring, etc. which have higher porality than a triazine ring are formed other than the triazine ring, so that it can be considered that dielectric characteristics (in particular, dielectric dissipation factor) are worsened. On the other hand, in the resin composition containing an epoxy resin containing a biphenyl structure of the present invention, a biphenyl group show hydrophobic property and low polarity, so that it can be considered that bad effects caused by using an epoxy resin in combination on dielectric characteristics are reduced than the conventional one. Also, a biphenyl skeleton with a brittle structure is introduced into the cured product, so that it can be considered that a temperature-dependency of dielectric characteristics is a little due to low mobility of a molecular chain particularly at a high temperature region. In addition, biphenyl skeleton-introduced system shows high strength and elongation at the glass state region and high elongation at a high temperature region as compared with the system in which the conventional epoxy resin is used in combination, so that a resin cured product excellent in crack resistance and extremely high heat resistance can be obtained.
Also, the resin compositions for printed wiring board according to the first invention and the second invention of the present invention are excellent in heat resistance under humid conditions and have high strength and elongation at a glass state region or elongation at high temperature region as compared with the case where a cyanate ester resin, etc. has used alone, or the case where a conventional epoxy resin is used in combination with the cyanate ester resin, etc., so that it is preferably used for a multi-layered printed wiring board of 10-layers or more, which is required to have crack resistance at drilling or reflow or to have severe heat resistance, to be used for laminated boards and prepreg use.